Old air in tanks.

[PfcAJ]

Isn't that how GUE et al. do it with tmx and standard mixes? EAN32 to a given pressure, top up with He?

You can do it either way, but helium first allows you to use lower pressures of helium.

With a booster it doesn't really matter, but with a cascade it does.

 

[Storker]

where do we get the tanks full of gin?

Home - Bombay Sapphire

Me, I'm more concerned about the source of tonic water. Schweppes can work in a pinch, but personally I prefer Fentiman's.

 

[TrimixToo]

You can do it either way, but helium first allows you to use lower pressures of helium.

With a booster it doesn't really matter, but with a cascade it does.

(Drifting ever further afield....) With my AG30, I find that I can pull the He bottle down better by far if I pump He first, O2 second, and air third when blending trimix. As He costs considerably more than O2, I'm happier returning an O2 tank with 150psi in it than an He tank; I try to pull the latter down under 100psi before returning. It probably helps that O2 pumps better (at least in my booster) than He.

(Back toward the thread's topic a bit....) It does take considerable time for a mix to mix thoroughly if left alone. I generally roll singles and move doubles up and down a few times. If I'm in a rush, I'll throw them in the car and drive around the block (my "block" is a couple of miles long). I'd bet that the mix that changed considerably without any signs of Bad Stuff in the Tank (tm) was analyzed after insufficient time or physical tank movement, or for too brief a time after an O2 top so that the content of the valve and dip tube still had mostly O2 in them, but what do I know? I wasn't there, so anything is possible.

(Even closer to the topic....) I have re-analyzed a number of tanks that have sat for well over a year with partial fills in them, to get more precise readings than the full percentage point markings I give them before diving. They have never been more than .4% away from the marked mix. This includes both lean (<21% O2) and rich (>21%) nitrox and trimix.

 

[Centrals]

It won't. But by all means, do the experiment.

BTW, here's a nice video showing that heavy molecules (in this case Br2, which has a molecular mass of 160, more than three and a half of that of CO2) certainly diffuses upwards in spite of the gravitational pull:

1. If the tank was filled with Helium first. Would the Br2 and He form a homogeneous mixture eventually?

 

[Centrals]

. And not that it matters, but CO2 is only 11x more massive than He.

This is embarrassing!!!!
Cheers.

 

[Storker]

1. If the tank was filled with Helium first. Would the Br2 and He form a homogeneous mixture eventually?

Yes

 

[grf88]

The issues the OP faced could be easily resolved if divers would require their air suppliers to provide them with the "best before date" of the air they are pumping. Many dive shops are reluctant to provide that information and will give all kinds of excuses why they can't provide it. Be insistent and make sure they give you that information along with the appropriate documentation.

 

[Peter69_56]

Or maybe he fills air first, then O2. Or either.

EDIT: I only blend continuous, but I can easily imagine myself filling a tank with air first, then O2. At least for me, the math is simpler that way.

O2 first then air, and yes PP either direct from cylinder or pumped. Only had it a few times where the 50Nx was 80 until mixed, I find it hard to get my head around the fact that it layers as looking at it simplistically one would expect a fair mix as the air rushes in after the O2 is put in, but at 100l/m at pressure, perhaps its just whisping in (effectively) so it doesnt disturb the O2 layered below (normally). I think the few times I had a high initial mix, the tank was originally empty after a clean, rather than at say 50 BAR and then topped up. Anyway I find it interesting, and just accept I should shake and roll after blending.

 

[Peter69_56]

The issues the OP faced could be easily resolved if divers would require their air suppliers to provide them with the "best before date" of the air they are pumping. Many dive shops are reluctant to provide that information and will give all kinds of excuses why they can't provide it. Be insistent and make sure they give you that information along with the appropriate documentation.

So what is best for? How does one determine with accuracy the best before date?

I have a cylinder of oxygen, whats its best before date?

 

[TrimixToo]

The issues the OP faced could be easily resolved if divers would require their air suppliers to provide them with the "best before date" of the air they are pumping. Many dive shops are reluctant to provide that information and will give all kinds of excuses why they can't provide it. Be insistent and make sure they give you that information along with the appropriate documentation.

Ah! A fisherman! We'll see how many you reel in this time (grin).